Temperatures rose into the mid-90s Wednesday, the fourth day of a heat wave, and National Weather Service forecasters warned that temperatures could rise towards 100 by the end of the workweek.

Forecasters cautioned Bay State residents that the heat would be combined with oppressive humidity. The hardest-hit part of the state Thursday is expected to be Southeastern Massachusetts, where temperatures in the mid-90s will combine with humidity to make it feel like it’s 107 degrees.

The weather will “create a dangerous situation in which heat illnesses are likely,” the National Weather Service said, issuing an excessive heat warning that advised people to avoid prolonged work in the sun or poorly ventilated areas, to drink plenty of fluids, to stay in air-conditioned areas, and to check on relatives and neighbors.

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With the state suffering in the grips of its third heat wave in two months, the mercury climbed to 94 degrees at 1 p.m. Wednesday at Logan International Airport. Wednesday night will see mostly dry conditions, with temperatures bottoming out in the mid-70s, according to the weather service.

“It looks like we are going to see temperatures in the mid-90s for Thursday and Friday,’’ said weather service meteorologist William Babcock. “But if the atmosphere gets a little warmer over the next couple of days, we could be up around 98 or 99.”

Saturday the heat will abate slightly. The chance for thunderstorms is expected to increase Thursday, with some severe storms possible on Friday and Saturday.

A cold front is likely to approach from the Great Lakes by Saturday night, and assert itself over the state by Sunday.

Starting Sunday and lasting until Tuesday, humidity will drop substantially, Babcock said. Temperatures will become seasonable starting Sunday, with highs in the mid-80s, although humidity and heat are likely to strike again Wednesday.

That could mean a fourth heat wave will develop late next week, the weather service said. A heat wave is officially defined as three days or more of temperatures of 90 or more.